1. Field of the Invention
This invention relates to a method for the production of a casting made of an aluminum-silicon alloy.
In aluminum-silicon castings, it is frequently desired to bring about a particularly fine-grained structure, at least at individual specific regions thereof. The fine-grained structure produces in a workpiece of this material characteristics of low thermal expansion, high tensile strength, and high resistance to fatigue. These characteristics may be achieved by annealing the finished casting which effects coagulation of the grains.
2. State of Prior Art
In aluminum-silicon castings silicon inclusions are not present in the form of separate particles but are interconnected by a dendritic network. In the event of the structural zone being externally loaded or deformed, this results in high peak stresses in the silicon which, even if the deformations are only relatively small, cause the brittle silicon inclusions to break and thus internal notches to be introduced into the material. The destruction of these unsuitable networks has so far only been possible by extremely long coagulation annealing of the castings. Such a method is described, for example, in German Patent Specification No. 12 34 399.
In order to obtain fine-grained structural zones in shaped castings, it is already known from British Patent Specification No. 1,337,731 to cool the wall areas of the casting in the molten state thereof by spraying water against the mould walls surrounding the casting. Such a measure causes the grain of the structure to become indeed smaller but only to a relatively narrowly limited extent. This is due to the fact that the solidifying melt shrinks in the mould, causing the contact between the mould wall and the casting to be lost and an air gap to be formed between the mould wall and the casting. This considerably reduces the heat transfer from the casting to the cooling medium, which has the effect that the heat required for the formation of a particularly fine-grained structure can no longer be removed from the casting to a sufficient extent.